Much attention is paid to organic light-emitting devices such as organic electrolurminescence (EL) devices usable for flat panel display devices. Specifically, promising as inexpensive, solid-emission-type, large-emission-area, full-color display devices and writing light source arrays, the organic light-emitting devices have been actively developed.
The organic light-emitting device generally comprises a couple of electrodes (a transparent electrode and a rear-surface electrode), and a light-emitting organic layer formed between the electrodes. When an electric field is applied to the organic light-emitting device between a pair of opposing electrodes, electrons are injected into the light-emitting device from the rear-surface electrode, while holes are injected thereinto from the transparent electrode. Electrons and holes are recombined in the light-emitting layer, and energy is emitted as light when an energy level is lowered from a conduction band to a valence band.
The organic layers in the organic light-emitting devices are mostly formed by a vapor deposition method. For instance, JP 9-167684 A and JP 2000-195665 A propose methods comprising uniformly forming an organic layer on a temporary support of mica or a film by a vapor deposition method, bringing the organic layer close to the substrate, and carrying out a heating vapor deposition. However, these methods are poor in productivity because they use a vapor deposition method. In addition, because only low-molecular-weight organic compounds can be used for organic layers in a vapor deposition method, the resultant organic light-emitting devices are insufficient in durability such as bending resistance, film strength, etc., when used for flexible displays, etc. This problem is serious particularly when they have large areas.
In view of the above problems in connection with the use of low-molecular-weight organic compounds, proposals have been made to provide high-molecular-weight organic EL devices comprising light-emitting thin-film layers made of high-molecular-weight compounds, or those composed of low-molecular-weight compounds dispersed in binder resins. For instance, Nature, Vol. 347, page 539, 1990 proposes a green-light-emitting, high-molecular-weight, organic EL device using poly(p-phenylenevinylene). The Japanese Journal of Applied Physics, Vol. 30, page L1938, 1991 proposes a high-molecular-weight, organic EL device using poly(3-alkylthiophene) for emitting red orange light. The Japanese Journal of Applied Physics, Vol. 30, page L1941, 1991 proposes a blue-light-emitting, high-molecular-weight, organic EL device using polyalkylfluorene. These high-molecular-weight devices are advantageous in making large-area, light-emitting devices, and their applications for flexible displays are expected. However, because the vapor deposition method cannot be used to form the organic light-emitting thin-film layers, thin-film layers are formed directly on substrates by wet methods.
The wet methods are, however, disadvantageous in that the formed organic layers are insufficient in the uniformity of thickness because of the surface tension of solutions, and that when a new liquid organic layer is laminated on the already solidified organic layer, the solidified organic layer tends to be dissolved in its interface with the new liquid organic layer. Accordingly, the organic electroluminescent devices obtained by the wet methods are poor in light-emitting uniformity, light-emitting efficiency and durability of devices.
WO 00/41893 discloses a method for thermally transferring an organic layer and a photo-thermal conversion layer onto a substrate by a laser beam by using a donor sheet having an organic layer and a photo-thermal conversion layer. Such a thermal transfer method is disadvantageous in that a gas often penetrates into an interface between the organic layer and the substrate, resulting in poor light-emitting properties. In addition, there is a problem that the light-emitting efficiency, durability and the uniformity of the organic EL device vary depending on the conditions of the interface of the organic layer.
In the case of thermal writing in a predetermined pattern using a thermal head or a laser common in printing technologies, a temperature distribution generated around an organic, thin-film pattern by thermal diffusion blurs its outline, failing to cut the organic, thin-film pattern from the donor accurately. Thus, organic light-emitting devices produced by this method are uneven in light emission and likely to suffer from poor durability, because of insufficient electric connection and the breakage of the organic layer. Further, yield is likely to be low because of low-accuracy positioning of the substrate and the thermal head or laser beam.